The present invention relates generally to switching systems and, more particularly, to an improved system and method for error-less switching from a working channel to an alternate channel carried over a diverse route.
Switching of digital signals from a service (working) channel to an alternate (protect) channel, and vice versa, in a telecommunications system can cause a xe2x80x9chitxe2x80x9d to the traffic. In other words, the payload can be corrupted during this switch from the working channel to the protect channel. This corruption occurs because the protect channel and the working channel have different signal payload pointer values, and a processor circuit must recognize the different pointer values and align its counter. During this time the traffic can be corrupted.
In some signal formats (for example, the SONET format), a payload and the starting location of a payload (i.e., a predetermined unit of data called a pointer value) can float within each frame and the bit streams arriving at the receiver from the working and protect channels may not be identical. Furthermore, the overhead bytes of the signal carried on the working channel may be different than the overhead bytes of the signal carried on the protect channel, even if the starting location of the payload and corresponding frames is the same. Signals carried on a SONET network may, for example, need to be re-routed from the working channel to an alternate protect channel which may pass through an intermediate central location. To keep the payload synchronous with the SONET network, the payload may be shifted within the frame. The signals that arrive at the receiving end on the working and protect channels could then possibly be different.
U.S. Pat. No. 5,051,979 claims to teach a method for achieving hit-less switching between SONET signals. In this method, each STS-1 signal is frame-aligned by an individual delay buffer and sent to an individual pointer processor for payload alignment. Inside the pointer processor, each signal""s payload data is extracted and inserted into a new frame, along with a new pointer value. At the output of each pointer processor, the new pointer values are sent to a pointer justification and controller circuit. One of the pointer processors is designated the master and the other pointer processor is designated the slave. The pointer justification and controller circuit monitors the pointer value generated by the master pointer processor and, based on the master pointer value, sends justification control information to the slave pointer processor. Thus, at the output of the two pointer processors, the pointer values match and the payloads are now aligned. Subsequently, each signal is sent to a 2:1 multiplexer which selects between the two STS-1 channels. Because the signals are both frame- and payload-aligned, a simple 2:1 selection can cause a switch from the working channel to the protect channel and back.
While this method appears to accomplish hit-less switching in switching systems employing a signal format such as the SONET format, the solution is complex and costly because it requires two complete pointer processor functions to accomplish payload alignment. This approach requires the generation of two distinct SONET frames before performing the hitless selection. The circuitry required to implement this solution is complex, requiring a separate pointer processor to regenerate a new SONET frame for each of the two channels before the switch can occur.
Additionally, it is possible in a SONET network for a signal""s pointer value to change abruptly in response to changes in the network (for example, when a faulty node returns to an operational state). In a hit-less switching application, it is important that abrupt changes in the pointer value of one channel (e.g., the protect channel) do not affect the other channel (e.g., the working channel).
Therefore, a need exists for an improved error-less (hit-less) switching technique that provides the capability to perform a hit-less switch between two signals in a telecommunications network using only a single pointer processor function to align the signal payloads and which is therefore simpler and less costly than present hit-less switching techniques.
A further need exists for an improved hit-less switching technique that does not require the generation of a distinct SONET frame for each of the two signals prior to the signal selection.
A still further need exists for an improved hitless switching technique with the ability to prevent abrupt changes in the pointer value of one channel (e.g., the protect channel) from affecting the other channel (e.g., the working channel).
The present invention provides an improved hit-less switching system and method that substantially eliminates or reduces disadvantages and problems associated with previously developed hit-less switching systems and methods used for error-less switching of signals from a working channel to an alternate protect channel.
In particular, the present invention provides a system and method for processing digital signals in a telecommunications system that allows for hit-less switching between a first digital signal in which a first payload, a first pointer value, and a first frame are transported on a first channel, and a second digital signal in which a second payload identical to the first payload, a second pointer value, and a second frame are transported on a second channel.
In one embodiment, the hit-less pointer aligner of the present invention includes first and second pointer followers, first and second comparators, first, second and third multiplexers, a delay buffer with control circuit, and a pointer generator. The system selects between working and protect STS-1 (SONET Synchronous Transport Level 1) signals, or other similar signals, without affecting their payload. At the circuit input, each STS-1 signal is processed by only one of either the first pointer follower or the second pointer follower, which receives the corresponding signal, extracts the corresponding pointer value, removes the corresponding frame and forwards the extracted pointer value to the first comparator and the corresponding payload to the second multiplexer. The first comparator receives the working pointer value and the protect pointer value and compares them to determine between the working signal and the protect signal which of the two signal""s payload is ahead and which is behind (the signal with the ahead payload is the ahead signal, and the other signal is the behind signal). The comparator generates a select bit which it forwards to the first, second and third multiplexers. The select bit is used by each of the first, second and third multiplexers to determine which of the two inputs each is receiving each will then forward (select).
The working and protect signals are also both forwarded unprocessed to a first multiplexer, bypassing the first and second pointer followers. The first multiplexer also receives as an input the select bit from the first comparator. Based on the select bit being low or high the first multiplexer determines which signal (working or protect) to select (i.e., let pass). The signal forwarded by the first multiplexer is the signal that is determined by the first comparator to be the behind signal and it is sent through the system unaffected.
The second 2:1 multiplexer receives as inputs from the working and protect pointer followers the working and protect signal payloads. Based on the value of the select bit forwarded by the first comparator, the second multiplexer selects the payload of the signal that was determined by the comparator to be ahead and forwards that ahead payload to the delay buffer.
The first and second pointer followers also forward to the third multiplexer the working and protect signal pointer values. Additionally, the third multiplexer receives as an input the select bit from the first comparator and uses it to determine which of the working and protect pointer values is the pointer value corresponding to the behind signal. The third multiplexer selects the pointer value associated with the behind signal and forwards it as a bypass pointer value to the second comparator.
After passing through the third multiplexer the ahead signal""s payload is written to the delay buffer, where an appropriate delay is built into the signal to match the ahead signal to the behind (bypass) signal. The delay buffer forwards the ahead signal payload to the pointer generator.
The pointer generator receives the ahead signal payload and generates a third frame and a PG pointer value for the ahead signal payload to create a third digital signal with a third payload that is identical to the first and second payloads and wherein the third frame may be aligned with the first and second frames. The circuit is fully symmetric so that a switch from protect traffic back to working traffic uses the same mechanism.
The pointer generator forwards the PG pointer value to the second comparator. The second comparator compares the PG pointer value with the bypass pointer value to determine the amount of delay between the working payload and the protect payload. The second comparator generates increment/decrement request signals that are forwarded to the delay buffer. The delay buffer varies the pointer value position of the payload in the third newly created digital signal in response to the increment/decrement request signals. The delay buffer control circuit further comprises a write counter in communication with the delay buffer to write the ahead signal payload bytes in sequence to the delay buffer, and a read counter in communication with the delay buffer to read the ahead signal payload bytes from the delay buffer.
The present invention provides an important technical advantage in that it implements hit-less switching between two channels in a telecommunications network without the added expense of the two full pointer processor functions of the aforementioned U.S. Pat. No. 5,051,979.
Another technical advantage of the present invention is that it provides an improved hit-less switching technique that can be used to perform a hit-less switch between two signals in a telecommunications network by aligning the signal payloads. Without the need for separate pointer processor functions to regenerate a new SONET frame for each of the two channels in the network following payload alignment, the system is simpler and less costly than present hit-less switching techniques.
Another technical advantage of the present invention is that it provides an improved hit-less switching technique with the ability to prevent abrupt changes in the pointer value of one channel (e.g., the protect channel) from affecting the other channel (e.g., the working channel).